Recently, in a one-box vehicle provided with a third seat in particular, in order to comply with a requirement for various seat arrangements, it is a remarkable trend to elongate the slide amount of a second seat (rear seat) in the vehicle front-rear direction. In order to cope with this, a technology has been disclosed that aims at elongating lower rails fixed to a vehicle floor and enabling rollers attached to upper rails to roll in the lower rails so that the seat slide amount can be elongated without being restrained in the slide amount by other respects than the length of the lower rails (refer to Patent Document 1).